1. Field of the Invention
The present invention relates to active pixel solid state photosensors and imagers using CMOS technology.
2. Description of the Related Art
Active pixel solid state sensors and devices for detecting electromagnetic radiation are well known and in widespread use. When mounted in camera systems, pixel arrays serve as vision or image sensors which produce electrical signals corresponding to the detected light levels. Examples of such photosensors are disclosed in EP739039 and in WO93/19489. These sensors, implemented using CMOS- or MOS-technology, utilize collection junctions which are regions adapted for collecting charges generated by radiation in the semiconductor substrate. The collection junctions are either p-n or n-p junctions, depending on whether the substrate is of p-type or n-type conductivity, respectively.
An active pixel is configured with circuitry integrated in the pixel to amplify the charge that is collected on the light sensitive element or component in the pixel. Active pixels may also be equipped with additional electronics for more elaborate functions, such as filtering, high speed operation, or operation in more extreme illumination conditions. Conversely, passive pixels do not have such circuitry, so they require charge-sensitive amplifiers which are connected to the pixel via a conductive wire or line of metallization. However, one primary drawback of active pixel CMOS or MOS sensors is that a significant part of the pixel surface is used for the detection circuitry, thereby limiting the collection area for each pixel.
Because all photon-generated charges within a recombination length from the collection junction have a chance of diffusing to, and being collected by, the junction, the charge sensitive volume of a collection junction is larger than the junction""s depletion layer. Based on this mechanism, a sensor with a small collection junction can have a larger photosensitive volume. For example, photosensors with an apparent front size or photosensitive region of approximately 30 xcexcm diameter can be made with junctions of 3 xcexcm by 2 xcexcm and with a recombination length of 15 xcexcm. However, for active pixels which contain other circuitry (e.g. detection circuitry), some charges that would otherwise have reached the collection junction are instead captured by the junctions or components of the additional circuitry. These charges taken by the pixel""s additional circuitry are therefore lost and do not contribute to the detected signal. This is a principal reason for the low fill factor or low sensitivity of active pixel sensors.
It is known in the art that photodiode dark current (i.e. current not caused by detected electromagnetic radiation) is primarily due to thermal generation of charge carriers at the edges of the photodiode, or at the interface between the silicon and SiO2. This dark current can be significantly reduced by a method called xe2x80x9cinversion modexe2x80x9d or xe2x80x9call phase pinningxe2x80x9d in which the Sixe2x80x94SiO2 interface is brought into inversion by applying a dopant layer to the surface of the photodiode. This dopant layer prevents contact between the buried channel (i.e. the useful collecting junction) and the Sixe2x80x94SiO2 interface. This method typically reduces the dark current two orders of magnitude.
An example of an active pixel device prior to the present invention is represented by Lee et al., U.S. Pat. No. 5,625,210 xe2x80x9cACTIVE PIXEL SENSOR INTEGRATED WITH A PINNED PHOTODIODExe2x80x9d, which illustrates integrating a n-well CMOS pinned photodiode with a transfer gate into an image sensing element of an active pixel element. As shown in FIG. 1, the p-type substrate 24 forms a p-n photodiode with the n-region 22 which becomes the photoactive element and stores the photoelectrons created by photons impinging onto the pixel. xe2x80x9cBuryingxe2x80x9d the n-well 22 under a p+ pinning region 20 confines the collected photoelectrons in the deeper n-region. Because the photodiode junction is then prevented from touching the Sixe2x80x94SiO2 interface 30, the generation of dark current at generation centers at the interface is suppressed. The electrostatic potential created by the pinning dopant region 20 also reduces the influence of any oxide layer charge on the junction potential. Such photodiodes also have better ionization radiation tolerances.
The pinning dopant region 20 of the photodiode also reduces the capacitance of the collection junction, which reduces the kTC noise of the sensor and the possibility of xe2x80x9cghostxe2x80x9d images; i.e. relics of prior frames"" bright images in subsequent dark frames. The so-called kTC noise, one of the primary sources of noise in imaging sensors, is typically expressed as an amount of noise charge (i.e. uncertainty of the measurements of the photo-generated charge), and it is proportional to the square root of the capacitance of the collection junction. Therefore any reduction of capacitance equates to a reduction of the kTC noise. The pinning dopant layer 20 reduces the capacitance of the collection junction by raising the minimum of the electrostatic potential well in which the photoelectrons are confined. When this potential well is shallower than the transfer bias of the transfer gate 28, the photodiode can be completely depleted or reset in a shorter amount of time. Therefore, with a sufficient reduction of the sensor""s capacitance by the pinning dopant layer 20, all of the photoelectrons can be transferred to the detection circuitry n-well 26 by turning on the transfer gate 28, leaving no charge in the potential well to contribute to a later frame""s image.
A MOSFET is formed by the transfer gate 28 above the p-type substrate 24 between the charge collection n-well 22 and the CMOS detection circuitry n-well 26. Application of a sufficient voltage to the transfer gate 28 forms a depletion region between the two n-wells 22 and 26, thereby providing an n-channel for charge transfer between the pinned photodiode and the floating diffusion CMOS detection circuitry. The transfer gate 28 is the gate or electrode that controls the conditional transfer of charges between a photodiode (or other structure containing charge, e.g. a storage gate) to a register. The sole function of the transfer gate 28 is as a switch, which creates the charge transfer channel when appropriately biased.
However, the present inventors recognized several disadvantages inherent in the pinned photodiode technology described in the prior art. First, precise manufacturing steps are crucial for proper performance of the pinned photodiode. While the pinning dopant region 20 and the n-well 22 are self-aligned with one another, a small misalignment between the transfer gate and the underlying n-wells 22 and 26 will drastically impair device performance. If the transfer gate 28 does not extend to the edges of both the n-wells 22 and 26, then there will be a p-type barrier to charge transfer, which can significantly reduce or eliminate the charge transfer, even when the transfer gate is fully biased. Second, some of the photoelectrons that otherwise would have reached the photodiode""s n-well 22 are instead collected by other circuitry (e.g. detection circuitry) in the proximity of the collection junction. In other words, the effective fill factor of the n-well 22 is rather limited because photoelectrons which would otherwise contribute to the signal are captured by the potential wells of the surrounding circuitry. And third, while low capacitance photodiodes have low kTC noise levels and high transfer efficiencies which minimize xe2x80x9cghostxe2x80x9d images, they also have small areas, and thus small light collection volumes.
The present invention provides an alternative device structure which overcomes the disadvantages of the devices described in the prior art while achieving some of their favorable characteristics. By utilizing a dual-purpose electrode (rather than a transfer gate) which extends beyond the edge of the collection photodiode, the present invention overcomes the sensitivity of device performance on small misalignments between manufacturing steps. Furthermore, the collection-mode potential of the dual-purpose electrode can be adjusted to achieve charge confinement and enhanced collection efficiency, reducing or eliminating the need for the additional p+ dopant layer found in the prior art. Finally, the present invention enhances the fill factor of the photodiode by shielding the photon-created charge carriers formed in the substrate from the potential wells of the surrounding circuitry.
One aspect of the present invention involves a detector of electromagnetic radiation. The detector has a semiconductor substrate with dopants of a first conductivity type at a first concentration density, and an insulating layer at its surface. A collection region with dopants of a second conductivity type opposite the first conductivity type at a second concentration density is formed in the surface region of the semiconductor substrate. A dual-purpose electrode is formed on the insulating layer, extending over both the surface of at least part of the collection region and over at least part of the substrate. Preferably, the collection region forms a junction with the semiconductor substrate. In one embodiment, the junction formed is a photodiode.
In one embodiment, the substrate further has a barrier region of the first conductivity type with a concentration density of dopants being higher than the concentration density of dopants in the substrate. In another embodiment, the barrier region extends at least partly under the dual purpose electrode. In yet a further embodiment, a detection region with dopants of the second conductivity type at a third concentration density is formed in the surface region of the semiconductor substrate, not bordering the collection region and being connected to read-out electronics.
In another embodiment, the surface regions of the semiconductor substrate beyond the collection region are barrier regions which have dopants of the first conductivity type at a concentration density larger than the concentration density of the semiconductor substrate, and the read-out electronics formed within shielding regions. Preferably, at least part of the charge carriers that are generated in the semiconductor substrate underneath the shielding regions are collected by the collection region.
In yet another embodiment, a pinning region with dopants of the first conductivity type at a fourth concentration density is within the surface region. Preferably, the pinning region is not covered by the dual purpose electrode. In one embodiment, the pinning region is aligned with the dual-purpose electrode, and extends along the collection region.
Another aspect of the present invention involves a method of making a detector of electromagnetic radiation. The method involves providing a semiconductor substrate with dopants of a first conductivity type at a first concentration density, and with an insulating layer at its surface, forming a collection region by introducing dopants of a second conductivity type which is opposite the first conductivity type at a second concentration density region into the surface region of the semiconductor substrate, and forming a dual-purpose electrode on the insulating layer with the dual-purpose electrode extending over the surface of the collection region.